As with all information storage devices, the recording surface of an optical disc must be protected from dust, small particles, or any other source of contamination. Since optical drives use removable media, such a protection is usually incorporated in the disc design. For instance, the widely used compact discs of various formats incorporate a layer of transparent plastic to separate the recording surface from the outside world. Such a surface in a 355.6 mm in diameter optical disc is protected by a polycarbonate transparent cover sheet. The latter is radially tensioned and sealed to a spacer and a perimeter ring at the inner and outer radii of the disc, respectively. The space under the cover sheet is filled with air enclosed there during assembly of the disc. Obviously, the barometric pressure of air under the cover sheet (equal to the air pressure in the clean room during assembly) and the atmospheric pressure of air surrounding the disc during drive operation may have different values. This will create a pressure differential for the air inside and outside the disc resulting in deflection of the cover sheet.
Due to centrifugal forces during disc rotation, the air under the cover sheet will be displaced toward the outside periphery of the disc. This air motion will result in additional deflection of the cover sheet. Its original surface will be transformed into a more complicated shape that may result in catastrophic conditions when the cover sheet touches the objective lens (at the outer radius) or the substrate (at the inner radius). For examples of optical discs with cover sheets, see commonly assigned U.S. Pat. Nos. 4,507,774 and 4,539,573.
Referring to FIGS. 1 and 2, a prior art two-sided optical disc has a substrate 10 which is protected from the outside world by transparent cover sheets 20 and 30. The disc is provided with a central opening 70. Each sheet is radially tensioned and sealed to spacers 40 or 50 and to a perimeter ring 60 at the inner and outer radii of the disc, respectively. Since the spacer thicknesses are larger than the height of the ring, the space under the cover sheet represents a frustum of a cone. Its volume is filled with air enclosed there during assembly of the disc.
As stated above, the barometric pressure of air under the cover sheet (equal to the air pressure in the clean room during assembly) and air surrounding the disc during drive operation may have different values. This will create some pressure differential for the air inside and outside the disc that will result in deflection of the cover sheet. Several graphs are shown in FIG. 3 illustrating the deformed cover sheet when the optical disc is placed at different altitudes. With increased altitudes above the sea level, the air pressure in the surrounding atmosphere is decreasing resulting in the cover sheet bulging. In such conditions, when the disc is used in the drive, the cover sheet may touch the objective lens of the optical head making reading or writing impossible. Similarly, at decreasing altitudes below the sea level, the air pressure of the surrounding atmosphere is increasing that deflects the cover sheet toward the substrate. If at high levels of pressure differentials the cover sheet touches the substrate, reading or writing becomes once again impossible. To avoid such catastrophic cases, the cover sheet deflection due to pressure differentials must be reduced or eliminated completely.
On top of that, when the disc is rotated in the optical drive, centrifugal forces will displace the air in the air space under the cover sheet toward the outside periphery of the disc. This will decrease the pressure of the entrapped air at the inner zone of the disc and increase the air pressure at the outer part of the disc. As a result, additional deflection of the cover sheet will occur transforming the latter into an S-shaped sheet. Such a case is illustrated in FIG. 4 where several graphs show the deflected shape of the cover sheet due to disc rotation at various angular velocities. With the increasing number of revolutions per minute (rpm) in disc rotation, the S-shaped cover sheet deflects at higher rates that may bring the cover sheet in contact with the substrate of the disc or the objective lens of the optical head.
Each graph in FIG. 4 was derived by an experimentally verified mathematical model developed to describe the cover sheet behavior at different conditions. As seen here, graphs for deflected cover sheet due to disc rotation intersect the original cover sheet of a stationary disc at the same so-called inflection point where on one lateral side of the point the cover sheet will bulge and on the other lateral side it will be depressed. At the inflection point, no deflection of the cover sheet occurs since at that point the air pressure has the same value if measured outside and inside the disc. This phenomena is observed only when the disc operation takes place at any altitude but with the air pressure inside and outside the disc statically equalized before rotation starts.
With the deflected cover sheet due to static pressure differentials, avoiding catastrophic cases of drive operation when the sheet touches the substrate or the objective lens may be achieved by lowering the difference in pressure values of the air entrapped under the cover sheet during the disc assembly and air surrounding the disc when it is used at various altitudes.